McMahon & Mann completed subsurface exploration and field and laboratory testing programs to characterize the site for an expansion to an existing wastewater treatment plant. The site is underlain by granular soil deposits with relatively high permeability. Construction of several tanks and a pump building required excavations extending 15 to 20 feet below the groundwater level.

We prepared foundation recommendation and excavation and dewatering recommendations for construction. For most structures, surrounding the excavations with a series of dewatering wells and sloping the excavations was sufficient. However, the deepest structure required tight sheeting driven to a lower permeability deposit below the granular zone to limit contaminated groundwater migration into the excavation.

McMahon & Mann prepared a conceptual design for the braced sheet pile cofferdam and provided the client with recommendations and comments on the specifications. We also reviewed and commented on the contractor’s plans for shoring the excavation and monitored its construction.